1. Field of the Invention
This invention relates to processes for the preparation of alkali metal dichromates and chromic acid by the electrolysis of monochromate and/or dichromate solutions in electrolytic cells in which the anode chamber and cathode chamber are separated by cation exchanger membranes.
2. Description of the Related Art
According to U.S. Pat. No. 3,305,463 and CA-A-739,447, the electrolytic preparation of alkali metal dichromate and chromic acid (CrO.sub.3) is carried out in electrolytic cells in which the electrode chambers are separated by a cation exchanger membrane.
For the production of alkali metal dichromates, alkali metal monochromate solutions or suspensions are introduced in the anode chamber and converted into alkali metal dichromate solutions by the selective transfer of alkali metal ions into the cathode chamber through the membrane. For the preparation of chromic acid, alkali metal dichromate or alkali metal monochromate solutions or a mixture of alkali metal dichromate and alkali metal monochromate solutions are introduced into the anode chamber and converted into solutions containing chromic acid. Sodium monochromate and/or sodium dichromate solutions are generally used for these processes. In both processes, an alkaline solution containing alkali metal ions is obtained in the cathode chamber. This solution may consist, for example, of an aqueous sodium hydroxide solution or, as described in CA-A-739 447, of an aqueous solution containing sodium carbonate.
For the production of alkali metal dichromate or chromic acid crystals, the solution formed in the anode chambers of the cells is concentrated by evaporation to enable the crystallization of sodium dichromate, for example, to take place at 80.degree. C. and the crystallization of chromic acid at 60.degree. to 100.degree. C. The crystallized products are separated off, optionally washed and dried.
When this process is carried out, compounds of polyvalent ions, in particular of alkaline earth ions, are deposited in the membrane and rapidly impair the functional efficiency of the membrane until it completely fails. The formation of these deposits is due to the presence of small amounts of polyvalent cations, in particular calcium and strontium ions, in the sodium dichromate and/or sodium monochromate solutions used as electrolytes such as those obtainable by the industrial processes described in Ullmanns Encyclopedia of Industrial Chemistry, 5th Edition, Volume A 7, 1986, pages 67-97.
It was an object of this invention to provide processes for the preparation of alkali metal dichromates and chromic acid by electrolysis which would be free from the disadvantages described above.